fix(volume-osd): keep the shell alive when nothing is visible

Quickshell exits once no window is visible. The OSD is hidden between
changes, and with no MPRIS player running there is nothing else to hold
it open, so the process loaded cleanly, logged no error and quit. The
symptom was a panel that never appeared: not a blank panel, a dead shell.

A 1x1 transparent window with an empty mask keeps it running. The mask
makes it click-through, so it cannot catch a stray click, and it draws
nothing.

This window existed earlier and was removed after a process check that
was measuring the wrong thing, which made it look unnecessary. The
README now says why it is there, so it does not get removed a third time.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G7NRsGyF9jGfPYS4zPqpN7

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